In the prior art, a sequence taken by a camera gives rise to a run of images being made where each image constitutes a distinct entity, and where the images are intended subsequently to be displayed in succession at a given rate that enables any animated movements that may have occurred in the scene while the sequence was being taken to be played back.
For example, when the scene is filmed by means of a television camera, signals representative of each image are obtained by scanning a photosensitive target that is included in a scanner and on which an image of the filmed scene is produced by the lens of the camera. This line-type scanning is performed at a rate and under conditions that are determined, and the resulting image signal is processed so as to enable it to be transmitted to remote apparatus that includes or constitutes an image receiver.
It is common practice to scan images at a rate of twenty-five or thirty images per second so as to enable any motion that may be taking place in the filmed scene to be picked up and subsequently played back realistically at a receiver apparatus. It is common practice to double the playback image rate in order to eliminate visual phenomena that may occur and that observers find troublesome, such as flicker. When images are played back by means of a television type apparatus, each reproduced image is commonly played back in two stages, as two successive fields, one field comprising the even scan lines on the screen of the receiver apparatus, and the other comprising the odd scan lines.
Motion playback implies that the receiver apparatus is capable of producing images from a run at a rate that corresponds to the rate at which the images were taken.
It is conventional to add synchronizing pulses to the image signal as provided by the camera, in particular to mark the beginnings of lines and the beginnings of image fields, thereby enabling the receiver apparatus to synchronize itself. When transmission is performed in such a manner as to enable substantially instantaneous display of the transmitted images, as is the case with television or with videophony, the receiver apparatus is commonly synchronized relative to a clock that governs image transmission at the source. Such synchronization can be performed, for example, by means of a digital phase-locked loop circuit when the images are transmitted synchronously and in digitized form.
Unfortunately, although that solution is usually satisfactory, it is not always applicable, particularly when the transmission between a source and a receiver apparatus is liable to take place over at least a portion of its path via means that do not guarantee proper transmission to the receiver apparatus of the synchronizing signals as generated by the clock of the source. This applies in particular when the succession of images making up a sequence coming from a source is transmitted, over at least a portion of its path, via a connection that is established via an asynchronous time-division network.
It is possible, particularly under such circumstances, for the digitized images of a sequence transmitted by a source to arrive at a rate that is only approximately the normal design rate for proper playback thereof by the receiver apparatus, even though the same images are indeed transmitted at the correct rate from the source.
It is also possible to envisage that images are transmitted by the source at a rate that is merely within some acceptable tolerance range without being exactly the rate that would normally be expected by the receiver apparatus.